Fully aromatic polyesters characterized by formation of anisotropic melts have been disclosed in a number of patents. These polyesters are generally spun at moderate molecular weight to form oriented fibers of low to moderate tensile strength. Subsequently, these as-spun fibers can be subjected to heat-treatment as a result of which they strengthen significantly. Aside from the expense associated with such heat-treatment, there is a tendency for filament sticking which must be carefully controlled. When attempts are made to spin higher molecular weight polymers of this class, poor properties and nonuniform fiber, e.g. excessive diameter fluctuations along the length of the filaments are encountered. In U.S. Pat. No. 4,612,154, a method for eliminating these fluctuations by proper polymer selection and processing techniques is described. The present invention permits the attainment of higher tensile strengths in as-spun fiber than can be attained with prior art processes.
U.S. Pat. No. 4,159,365 discloses optically anisotropic melt-forming polyesters and fibers based on phenylhydroquinone terephthalate containing 0-10 mol % (based on total mols of units) of other aromatic or cycloaliphatic polyester forming units. One of the polymers exemplified contains 8.1 mol % (as defined above) of p-hydroxy benzoic acid units and is polymerized to an inherent viscosity, .eta..sub.inh, of 2.12. The as-spun fiber exhibited a tensile strength of 6.1 grams per denier (gpd). Heat strengthening was required to obtain high strength yarns.